Controlling the helicity of 2,2′-bipyridyl ruthenium(II) and zinc(II) hemicage complexes

Article abstract of DOI:10.1021/ja067016v

Two enantiomers of a new 4,5-pineno-2,2′-bipyridine ligand were synthesized and subsequently incorporated into hemicage ligands through a phenyl linker to yield ligands (+)-L₁ and (-)-L₁ or through a mesityl linker to yield ligands (+)-L₂ and (-)-L₂. Complexation of these ligands to Ru(II) afforded diastereomerically pure Δ and Λ isomers, as verified through circular dichroism and circularly polarized luminescence spectroscopy. Ligands (+)-L₂ and (-)-L ₂ were further coordinated to Zn(II) to form a complex with intriguing photophysical properties. Whereas Zn(bpy)₃²⁺ was shown to be a fluorescent emitter outside the visible spectrum, the caging process provided an unprecedented enhancement of intersystem crossing and subsequent switching to the phosphorescent emission of blue light. Additionally, the chiroptical properties of the Zn(II) complexes were also studied.

Full text of DOI:10.1021/ja067016v

Published on Web 12/09/2006
Controlling the Helicity of 2,2′-Bipyridyl Ruthenium(II) and
Zinc(II) Hemicage Complexes
Karl D. Oyler, Frederick J. Coughlin, and Stefan Bernhard*
Contribution from the Department of Chemistry, Princeton UniVersity,
Princeton, New Jersey 08544
Received September 29, 2006; E-mail: bern@princeton.edu
Abstract: Two enantiomers of a new 4,5-pineno-2,2′-bipyridine ligand were synthesized and subsequently
incorporated into hemicage ligands through a phenyl linker to yield ligands (+)-L1 and (-)-L1 or through a
mesityl linker to yield ligands (+)-L2 and (-)-L2. Complexation of these ligands to Ru(II) afforded
diastereomerically pure ∆ and Λ isomers, as verified through circular dichroism and circularly polarized
luminescence spectroscopy. Ligands (+)-L2 and (-)-L2 were further coordinated to Zn(II) to form a complex
with intriguing photophysical properties. Whereas Zn(bpy)32+ was shown to be a fluorescent emitter outside
the visible spectrum, the caging process provided an unprecedented enhancement of intersystem crossing
and subsequent switching to the phosphorescent emission of blue light. Additionally, the chiroptical properties
of the Zn(II) complexes were also studied.
Introduction
In the past, complicated and time-consuming methods such
as cocrystallization6 or chiral solid-phase HPLC were required
Enantiopure transition metal complexes have been and
continue to be a subject of great interest because of their
applicability toward such diverse areas as catalysis,1 materials,2
and chemical biology.3 As a consequence of their typically
charged nature, metal complex enantiomers and diastereomers
are most commonly resolved through cocrystallization with
chiral counterions, a fastidious methodology that was pioneered
by Werner in 1911.4 In contrast, direct stereoselective synthesis
of metal complexes was largely unexplored throughout most
of the 20th century. Over the past decade, however, much more
attention has been paid to deliberate control of the helicity of
metal complexes through synthetically tailored ligand systems.
Octahedral complexes of 2,2′-bipyridine (bpy) or 2-phenylpy-
ridine (ppy) containing d6 metals such as Ru(II), Os(II), and
Ir(III) have been the focus of considerable research because of
their established light-emitting and electron-transferring proper-
ties. As a result of the bidentate nature of these pyridyl ligands,
the octahedral complexes formed from them will be one of two
optical isomers, ∆ or Λ; in the case of bidentate ligands of Cs
symmetry such as phenylpyridine, facial (fac) and meridional
(mer) isomers will also be present. For supramolecular as-
semblies possessing multiple metal centers coordinated to these
types of ligands, such a variety of possible isomers can present
a serious problem in that the characterization of these complexes
through traditional means becomes difficult, if not impossible.5
to separate the ∆ and Λ isomers of metal complexes.7-9 Further,
even after being resolved, these species are susceptible to
photoracemization, which may restore the racemic mixture over
time and ruin the separation. To circumvent these problems,
recent methods have centered on the use of caged ligand
structures possessing chiral subsitituents.10,11 Caged ligand
structures present a number of advantages; in particular, their
substitutional inertness makes them ideal for synthesizing metal
complexes that are typically highly labile in solution, such as
tris-diimine complexes of Fe(II) or Zn(II). Additionally, the rigid
structure provided by caged ligands can greatly assist in reducing
the rate constant for nonradiative decay (knr) and thus improve
the quantum efficiency of luminescent metal complexes.12,13
Stereochemically, caged ligand systems are capable of
predetermining fac or mer isomerism, but are not selective
between the ∆ and Λ isomers; for this to occur, further
modification is necessary. In the recent past, this has been
accomplished through the use of terpene-based substituents,
most often pinene, as first implemented by Hayoz and von
Zelewsky.14 The chiral, steric bulk of such substituents can
(7) Rutherford, T. J.; Pellegrini, P. A.; Aldrich-Wright, J.; Junk, P. C.; Keene,
F. R. Eur. J. Inorg. Chem. 1998, 1677-1688.
(8) Fletcher, N. C.; Nieuwenhuyzen, M.; Rainey, S. J. Chem. Soc., Dalton
Trans. 2001, 2641-2648.
(9) Hesek, D.; Inoue, Y.; Everitt, S. R. L.; Ishida, H.; Kunieda, M.; Drew,
M. G. B. Inorg. Chem. 2000, 39, 308-316.
(10) Hamann, C.; von Zelewsky, A.; Neels, A.; Stoeckli-Evans, H. J. Chem.
Soc., Dalton Trans. 2004, 402-406.
(1) Inoue, Y.; Ramamurthy, V. Chiral Photochemistry; Marcel Dekker: New
York, 2004; pp 261-313.
(2) Mamula, O.; von Zelewsky, A. Coord. Chem. ReV. 2003, 242, 87-95.
(3) Myari, A.; Hadjiliadis, N.; Garoufis, A. J. Inorg. Biochem. 2005, 99, 616-
626.
(4) Werner, A. Chem. Br. 1911, 44, 1887-1890.
(5) von Zelewsky, A.; Mamula, O. J. Chem. Soc., Dalton Trans. 2000, 219-
231.
(6) Sagu¨e´s, J. A. A.; Gillard, R. D.; Smalley, D. H.; Williams, P. A. Inorg.
Chim. Acta 1980, 43, 211-216.
(11) Schaffner-Hamann, C.; von Zelewsky, A.; Barbieri, A.; Barigelletti, F.;
Muller, G.; Riehl, J. P.; Neels, A. J. Am. Chem. Soc. 2004, 126, 9339-
9348.
(12) Barigelletti, F.; De Cola, L.; Balzani, V.; Belser, P.; von Zelewsky, A.;
Voegtle, F.; Ebmeyer, F.; Grammenudi, S. J. Am. Chem. Soc. 1989, 111,
4662-4668.
(13) Beeston, R. F.; Aldridge, W. S.; Treadway, J. A.; Fitzgerald, M. C.; DeGraff,
B. A.; Stitzel, S. E. Inorg. Chem. 1998, 37, 4368-4379.
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9
210
J. AM. CHEM. SOC. 2007, 129, 210-217
10.1021/ja067016v CCC: $37.00 © 2007 American Chemical Society

Controlling Helicity of Ru(II) and Zn(II) Complexes
A R T I C L E S
3H), 1.78 (d, J ) 10.07 Hz, 1H), 1.84 (t, J ) 10.84 Hz, 1H), 2.08 (m,
2H), 2.29 (m, 1H), 2.37 (t, J ) 5.34 Hz, 1H), 2.62 (m, 1H), 3.32 (m,
1H), 3.42 (m, 1H), 3.53 (m, 1H), 3.65 (m, 1H), 4.81 (d, J ) 3.05 Hz,
1H). 13C NMR (125 MHz, CDCl3): δ 15.20, 15.42, 22.10, 22.20, 24.27,
26.10, 40.33, 40.77, 48.40, 58.17, 63.42, 65.08, 103.70, 212.81.
(1R,5R)-3-(Diethoxymethyl)nopinol (2). Under nitrogen atmo-
sphere, 1 (8.00 g, 0.033 mol) was dissolved in 100 mL of anhydrous
tetrahydrofuran and cooled to -78 °C. Lithium aluminum hydride (1.0
M in hexane, 40.0 mL, 0.043 mol) was added dropwise over 20 min.
The mixture was then stirred for 1 h. The reaction was quenched through
the slow addition of 20 mL of glacial acetic acid. The entire mixture
was poured into 500 mL of hexanes, and an additional 400 mL of water
was added. The layers were then separated, and the aqueous portion
was extracted with hexanes (3 × 100 mL). The combined organic layers
were dried with anhydrous sodium sulfate, and the solvent was removed
by rotary evaporation to yield 7.40 g (94%) of 2. 1H NMR (400 MHz,
CDCl3): δ 0.63 (d, J ) 10.07 Hz, 1H), 1.04 (s, 3H), 1.18 (m, 9H),
1.50 (m, 1H), 1.89 (s, 1H), 1.97 (m, 1H), 2.05 (m, 1H), 2.25 (m, 2H),
3.54 (m, 3H), 3.77 (m, 1H), 3.87 (m, 1H), 4.28 (d, J ) 7.90 Hz, 1 H).
The instability of 2 necessitated the next step to be carried out
immediately, and 13C NMR was not acquired.
directly lead to the preferential synthesis of a particular
stereoisomer. Such caged metal complexes combine substitu-
tional inertness with predetermined configuration at the metal
center, thus making them ideal candidates for applications such
as chromophores in stereogenic displays15 or inducing chirality
in other metal complexes through stereoselective photoinduced
electron-transfer reactions.16
Here, we report on the synthesis of a new, pinene-substituted
2,2′-bipyridine ligand and its enantiomer with applicability
toward fields such as asymmetric catalysis and the stereoselec-
tive formation of luminescent metal complexes. Through
stereospecific alkylation of these ligands with phenyl or mesityl
linking groups, we successfully synthesized hemicaged com-
plexes of Ru(II) and blue-emitting Zn(II) with complete
synthetic control of the helicity at the metal center as determined
through circular dichroism (CD) and circularly polarized
luminescence (CPL) spectroscopy.
Experimental Section
General. Reagents and solvents were purchased from Aldrich and
used as received. The following were synthesized as described in the
literature: 1-(2-acetylpyridyl)pyridinium bromide,17 (1R,5R)-(+)-â-
pinene,18 1,3,5-tris(bromomethyl)benzene,19 (1S,5R)-(-)-nopinone and
(1R,5S)-(+)-nopinone,20 5-methyl-2,2′-bipyridine,21 Ru(DMSO)4Cl2,22
[Ru(bpy)3]2+(PF6-)2,23 and [Zn(bpy)3]2+(PF6-)2.24 Varian/INOVA 400
(1S,5R)-Isomyrtenal (3). After 2 (7.40 g, 0.031 mol) was dissolved
in 75 mL of tetrahydrofuran and 15 mL of 1 M hydrochloric acid, the
solution was heated under reflux overnight. The reaction mixture was
then poured into 150 mL of saturated aqueous sodium bicarbonate
solution, followed by extraction (3 × 50 mL) with diethyl ether. The
organic layer was dried with anhydrous sodium sulfate and rotary
and 500 MHz spectrometers were used to record 1H NMR spectra; 13
C
1
evaporated to yield 4.00 g (87%) of yellow, liquid product. H NMR
NMR data were obtained exclusively on the 500 MHz spectrometer
(acquired at 125 MHz). A Hewlett-Packard 5898B (Electrospray) MS
engine spectrometer was used to measure the mass spectra. Elemental
analyses (CHN) were conducted by the Microanalytical Laboratory at
the University of Illinois, Urbana-Champaign.
(500 MHz, CDCl3): δ 0.75 (s, 3H), 1.30 (s, 3H), 2.21 (m, 1H), 2.45
(m, 3H), 7.32 (m, 1H), 9.42 (s, 1H). 13C NMR (125 MHz, CDCl3): δ
21.48, 26.25, 28.41, 31.47, 39.52, 39.79, 43.28, 139.11, 161.10, 193.28.
(7R,9R)-4,5-Pineno-2,2′-bipyridine (4). The ligand was prepared
via a Kro¨hnke bipyridine synthesis.26 Ammonium acetate (2.05 g, 0.027
mol), 1-(2-acetylpyridyl)pyridinium bromide (3.54 g, 0.013 mol), and
3 (1.91 g, 0.013 mol) were dissolved in 150 mL of methanol and heated
under reflux overnight. The reaction mixture was poured into 150 mL
of hexanes, and 150 mL of water was added. The layers were separated,
and the aqueous portion was extracted with hexanes (4 × 50 mL). The
combined organic layers were dried with anhydrous sodium sulfate,
and the solvent was removed by rotary evaporation to yield 1.62 g
∆-Hemicage Complexes. (1R,5R)-3-(Diethoxymethyl)nopinone
(1). The synthesis was carried out according to the procedure described
by Mock and Tsou.25 Redistilled boron trifluoride diethyl etherate (13.5
mL, 0.110 mol) dissolved in 80 mL of dichloromethane was added
dropwise over 20 min to triethyl orthoformate (14.9 mL, 0.090 mol)
under nitrogen atmosphere at -30 °C. The mixture was then allowed
to warm to 0 °C. After being stirred for 15 min, the mixture was cooled
to -78 °C and 6.20 g (0.045 mol) of (1R,5S)-(+)-nopinone was added,
followed by dropwise addition of N,N-diisopropylethylamine (23.5 mL,
0.135 mol). After being stirred for 1 h, the reaction mixture was poured
into 600 mL of saturated aqueous sodium bicarbonate solution. An
additional 250 mL of dichloromethane was then added, and the mixture
was stirred for 10 min at room temperature. The organic layer was
separated and washed with cold 1 M sulfuric acid followed by cold
water. The organic phase was then dried with anhydrous sodium sulfate,
and the solvent was removed by rotary evaporation to yield 8.66 g
(80%) of orange, liquid product. 1H NMR (500 MHz, CDCl3): δ 0.76
(s, 3H), 1.01 (t, J ) 7.02 Hz, 3H), 1.11 (t, J ) 7.02 Hz, 3H), 1.20 (s,
1
(46%) of off-white crystals. H NMR (500 MHz, CDCl3): δ 0.70 (s,
3H), 1.28 (d, J ) 9.46 Hz, 1H), 1.43 (s, 3H), 2.40 (m, 1H), 2.71 (m,
1H), 2.91 (t, J ) 5.49 Hz, 1H), 3.05 (s, 2H), 7.30 (m, 1H), 7.81 (m,
1H), 8.01 (s, 1H), 8.38 (d, J ) 7.94 Hz, 1H), 8.42 (s, 1H), 8.68 (m,
1H). 13C NMR (125 MHz, CDCl3): δ 21.58, 26.33, 30.51, 31.45, 39.16,
40.47, 47.72, 118.81, 121.24, 123.55, 131.42, 137.13, 148.28, 149.28,
153.69, 156.94, 157.19.
[(7R,9R,10S)-pbpyCH2]3Ph (pbpy ) 4,5-Pineno-2,2′-bipyridine)
((-)-L1). Under nitrogen atmosphere, 4 (1.00 g, 0.004 mol) was
dissolved in 60 mL of anhydrous tetrahydrofuran and cooled to 0 °C.
Lithium diisopropylamide (1.8 M in heptane/THF/ethylbenzene, 4.44
mL, 0.008 mol) was added dropwise over 15 min, and the resulting
black solution was stirred for an additional 30 min. 1,3,5-Tris-
(bromomethyl)benzene (0.280 g, 7.84 × 10-4 mol) was added in small
portions over 1 h, and the solution was stirred overnight. The reaction
mixture was then poured into 200 mL of 10% ammonium chloride
solution. The layers were separated, and the aqueous portion was
extracted (3 × 75 mL) with diethyl ether. The combined organic
fractions were dried with anhydrous sodium sulfate, and the solvent
was removed by rotary evaporation to give an orange oil. The product
was purified by column chromatography on silica gel using a 3:2 ethyl
acetate/hexanes eluent to give 0.170 g (24%) of solid, yellow product.
1H NMR (500 MHz, CDCl3): δ 0.63 (s, 3H), 1.39 (s, 3H), 1.47 (d,
J ) 9.77 Hz, 1H), 2.07 (s, 1H), 2.58 (m, 1H), 2.81 (t, J ) 11.60 Hz,
(15) Chen, S. H.; Katsis, D.; Schmid, A. W.; Mastrangelo, J. C.; Tsutsui, T.;
Blanton, T. N. Nature 1999, 397, 506-508.
(16) Hamada, T.; Ohtsuka, H.; Sakaki, S. J. Chem. Soc., Dalton Trans. 2001,
928-934.
(17) Mu¨rner, H.; von Zelewsky, A.; Stoeckli-Evans, H. Inorg. Chem. 1996, 35,
3931-3935.
(18) Brown, H. C.; Zaidlewicz, M.; Bhat, K. S. J. Org. Chem. 1989, 54, 1764-
1766.
(19) Ilioudis, C. A.; Tocher, D. A.; Steed, J. W. J. Am. Chem. Soc. 2004, 126,
12395-12402.
(20) Brown, H. C.; Weissman, S. A.; Perumal, P. T.; Dhokte, U. P. J. Org.
Chem. 1990, 55, 1217-1223.
(21) Polin, J.; Schmohel, E.; Balzani, V. Synthesis 1998, 321-324.
(22) Evans, I. P.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1973,
204-209.
(23) Bernhard, S.; Barron, J. A.; Houston, P. L.; Abrun˜a, H. D.; Ruglovksy,
J. L.; Gao, X.; Malliaras, G. G. J. Am. Chem. Soc. 2002, 124, 13624-
13628.
(24) Chlistunoff, J. B.; Bard, A. J. Inorg. Chem. 1993, 32, 3521-3527.
(25) Mock, W. L.; Tsou, H. R. J. Org. Chem. 1981, 46, 2557-2561.
(26) Kro¨hnke, F. Synthesis 1976, 1-24.
9
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A R T I C L E S
Oyler et al.
1H), 2.93 (t, J ) 5.04 Hz, 1H), 3.39 (m, 2H), 7.01 (s, 1H), 7.30 (m,
1H), 7.83 (t, J ) 7.78 Hz, 1H), 8.04 (s, 1H), 8.40 (d, J ) 7.93 Hz,
1H), 8.62 (s, 1H), 8.69 (d, J ) 3.97 Hz, 1H). 13C NMR (125 MHz,
CDCl3): δ 21.18, 26.61, 27.63, 40.22, 41.13, 41.15, 43.62, 48.26,
118.66, 121.33, 123.67, 128.23, 134.86, 137.16, 140.60, 148.00, 149.33,
154.04, 156.80, 156.96. [R]D -34° (25 °C, 5 mg in 5 mL of CHCl3).
[(7R,9R,10S)-pbpyCH2]3Ph(CH3)3 ((-)-L2). The ligand was pre-
pared via the procedure identical to that for (-)-L1, substituting 0.638
g (1.60 mmol) of tris(bromomethyl)mesitylene for 1,3,5-tris(bromo-
methyl)benzene and reacting with 4 (2.00 g, 8.00 mmol) and lith-
ium diisopropylamide (1.8 M in heptane/THF/ethylbenzene, 4.00 mL,
8.00 mmol) to yield 0.516 g (36%) after purification. 1H NMR
(500 MHz, CDCl3): δ 0.61 (s, 9H), 1.35 (s, 9H), 1.59 (d, J ) 9.77 Hz,
3H), 2.03 (m, 3H), 2.44 (s, 9H), 2.66 (m, 3H), 2.93 (t, J ) 5.34 Hz,
2.44 (m, 3H), 2.94 (m, 3H), 3.16 (m, 9H), 3.62 (t, J ) 8.25 Hz, 3H),
7.53 (s, 3H), 7.61 (m, 3H), 7.72 (d, J ) 4.27 Hz, 3H), 8.30 (t, J )
7.63 Hz, 3H), 8.38 (s, 3H), 8.66 (d, J ) 8.24 Hz, 3H). 13C NMR (125
MHz, d6-acetone): δ 17.54, 20.34, 25.59, 28.00, 35.75, 37.55, 42.16,
46.98, 48.78, 120.49, 123.32, 127.53, 135.18, 136.63, 136.88, 141.93,
146.93, 147.02, 147.94, 150.05, 162.37. MS (m/z; ESI): 1117 (2%,
M2+ - PF6-), 485 (100%, M2+ - 2PF6-). Anal. Calcd for C63H66N6-
Zn(PF6)2‚2H2O: C, 58.27; H, 5.43; N, 6.47. Found: C, 58.48; H, 5.17;
N, 6.39.
Λ-Hemicage Complexes. The (+)-L1 and (+)-L2 ligands were
synthesized starting from (1S,5R)-(-)-nopinone and otherwise prepared
in a fashion similar to the procedures described above. Λ-Ru(II)
complexes were prepared from both (+)-L1 and (+)-L2, while a Λ-Zn-
(II) complex was prepared from (+)-L2.
9H), 3.15 (m, 3H), 3.30 (d, J ) 10.99 Hz, 3H), 3.39 (dd, J1
)
Racemic Hemicages. [bpy(CH2)2]3Ph(CH3)3 (L3). 5-Methyl-2,2′-
bipyridine (4.14 g, 0.0245 mol) was dissolved with stirring in 100 mL
of anhydrous THF at -78 °C under N2. Lithium diisopropylamide (2.0
M in heptane/THF/ethylbenzene, 12.25 mL, 0.0245 mol) was added
dropwise by syringe over 30 min; the resulting black solution was then
stirred for an additional 15 min. Tris(bromomethyl)mesitylene (1.95
g, 0.00490 mol) was dissolved in 50 mL of anhydrous THF and added
dropwise by syringe over 3 h. After being stirred for an additional hour,
the black solution was poured into 500 mL of hexanes, forming a cloudy
blue mixture that gradually turned yellow. After being filtered, the
yellow solid was washed with hexanes and then dissolved in 500 mL
of CH2Cl2. The solution was washed with 10% NH4Cl solution and
backwashed with water, and then the combined organic layers were
dried over Na2SO4 (s) and concentrated to dryness by rotary evaporation
to yield 2.90 g (89%) of yellow, solid product of sufficient purity to
carry out metal complexation reactions. For characterization experi-
ments, the ligand was further purified by recrystallization from hexanes/
4.12 Hz, J2 ) 14.19), 7.31 (m, 3H), 7.82 (t, J ) 7.63 Hz, 3H), 8.06
(s, 3H), 8.41 (d, J ) 7.63 Hz, 3H), 8.65 (s, 3H), 8.68 (m, 3H). 13C
NMR (125 MHz, CDCl3): δ 21.01, 26.61, 29.03, 34.76, 41.08, 41.14,
43.82, 48.27, 118.69, 121.40, 123.74, 134.18, 135.08, 135.85, 137.21,
147.65, 147.78, 149.32, 156.45, 156.66. [R]D -183° (25 °C, 5 mg in
5 mL CHCl3).
∆-[Ru((-)-L1)](PF6)2 (7). (-)-L1 (0.100 g, 0.116 mmol) was
dissolved in 100 mL of hot ethanol, and Ru(DMSO)4Cl2 (0.056 g, 0.116
mmol) was added in small portions over 30 min. The red solution was
heated under reflux for 3 h before removal of the solvent by rotary
evaporation. The resulting red solid was then dissolved in 50 mL of
water, and a solution of ammonium hexafluorophosphate (1.00 g) in 5
-
mL of water was added to precipitate the ruthenium complex as a PF6
salt. The complex was separated by filtration, dissolved in acetone,
and filtered through a short column of aluminum oxide to remove
polymeric impurities. Following evaporation of the solvent from the
filtrate, recrystallization by vapor diffusion of acetonitrile and ether
yielded 0.030 g (21%) of pure product. 1H NMR (500 MHz, d6-
acetone): δ 0.36 (s, 9H), 1.48 (s, 9H), 1.86 (d, J ) 10.38 Hz, 3H),
2.40 (m, 3H), 2.87 (m, 6H), 3.12 (t, J ) 5.49 Hz, 3H), 3.29 (m, 3H),
3.67 (m, 3H), 7.36 (s, 3H), 7.41 (d, J ) 5.19 Hz, 3H), 7.47 (m, 1H),
7.76 (s, 3H), 8.13 (t, J ) 7.64 Hz, 3H), 8.40 (s, 3H), 8.69 (d, J ) 8.24
Hz, 3H). 13C NMR (125 MHz, d6-acetone): δ 20.30, 25.55, 28.22,
39.23, 41.47, 41.85, 47.01, 48.50, 121.05, 124.16, 127.31, 130.91,
137.62, 138.06, 140.63, 150.09, 151.00, 154.51, 157.87, 158.34. MS
(m/z; ESI): 1111 (1%, M2+ - PF6-), 483 (100%, M2+ - 2PF6-). Anal.
Calcd for C60H60N6Ru(PF6)2‚2H2O: C, 55.77; H, 4.99; N, 6.50.
Found: C, 55.97; H, 5.00; N, 6.13.
1
dichloromethane. H NMR (400 MHz, CDCl3): δ 2.37 (s, 9H), 2.84
(m, 6H), 3.05 (m, 6H), 7.30 (m, 3H), 7.68 (dd, J1 ) 8.13 Hz, J2 )
2.24 Hz, 3H), 7.82 (td, J1 ) 7.55 Hz, J2 ) 1.81 Hz, 3H), 8.37 (m, 6H),
8.56 (s, 3H), 8.69 (m, 3H). 13C NMR (125 MHz, CDCl3): δ 16.25,
32.73, 33.00, 121.15, 123.82, 132.59, 136.39, 137.10, 137.27, 137.81,
149.33, 149.37, 154.29, 156.26.
∆/Λ-[Ru(L3)](PF6)2 (11). The complex was prepared via a procedure
identical to that for 7. L3 (0.100 g, 0.149 mmol) and Ru(DMSO)4Cl2
(0.072 g, 0.149 mmol) were combined to give 0.035 g (22%) of product.
1H NMR (500 MHz, d6-acetone): δ 1.96 (s, 9H), 2.77 (m, 3H), 2.92
(m, 3H), 3.07 (m, 3H), 3.31 (m, 3H), 6.49 (d, J ) 1.46 Hz, 3H), 7.51
(td, J1 ) 6.23 Hz, J2 ) 1.46 Hz, 3H), 8.06 (d, J ) 5.49 Hz, 3H), 8.12
(td, J1 ) 7.69 Hz, J2 ) 1.46 Hz, 3H), 8.28 (dd, J1 ) 8.42 Hz, J2 )
1.83 Hz, 3H), 8.65 (t, J ) 8.06 Hz, 6H). 13C NMR (125 MHz, d6-
acetone): δ 16.92, 29.50, 30.89, 124.29, 127.74, 134.06, 134.85, 138.01,
138.55, 141.08, 151.74, 151.88, 154.96, 157.30. MS (m/z; ESI): 913
(5%, M2+ - PF6-), 384 (100%, M2+ - 2PF6-). Anal. Calcd for
C45H42N6Ru(PF6)2‚2H2O: C, 49.41; H, 4.24; N, 7.68. Found: C, 49.68;
H, 4.16; N, 7.68.
∆-[Ru((-)-L2)](PF6)2 (8). The procedure identical to that for 7 was
used, substituting 0.100 g (0.110 mmol) of ligand (-)-L2 in place of
(-)-L1. Complexation with Ru(DMSO)4Cl2 (0.053 g, 0.110 mmol) and
recrystallization yielded 0.039 g (27%). 1H NMR (500 MHz, d6-
acetone): δ 0.34 (s, 9H), 1.49 (s, 9H), 2.02 (d, J ) 10.38 Hz, 3H),
2.23 (s, 9H), 2.45 (m, 3H), 2.97 (m, 3H), 3.15 (m, 9H), 3.62 (t, J )
8.24 Hz, 3H), 7.35 (s, 3H), 7.47 (m, 3H), 7.50 (d, J ) 4.58 Hz, 3H),
8.13 (t, J ) 7.63 Hz, 3H), 8.40 (s, 3H), 8.67 (d, J ) 8.24 Hz, 3H). 13
C
∆/Λ-[Zn(L3)](PF6)2 (12). The complex was prepared by dissolving
L3 (0.100 g, 0.150 mmol) in hot ethanol (200 mL) followed by the
addition of NH4PF6 (0.200 g) and ZnCl2 (0.020 g, 0.130 mmol). After
being refluxed overnight, 100 mL of solution was removed by rotary
evaporation, leading to the precipitation of white product. Subsequent
filtering and recrystallization via vapor diffusion of acetonitrile/ether
NMR (125 MHz, d6-acetone): δ 17.66, 20.29, 25.60, 28.08, 35.81,
37.53, 42.46, 46.93, 48.45, 121.17, 124.13, 127.35, 135.05, 136.60,
137.51, 137.61, 149.66, 151.10, 151.10, 154.56, 158.04, 158.13. MS
(m/z; ESI): 1155 (4%, M2+ - PF6-), 504 (100%, M2+ - 2PF6-). Anal.
Calcd for C63H66N6Ru(PF6)2‚2H2O: C, 56.71; H, 5.29; N, 6.30.
Found: C, 56.69; H, 5.27; N, 6.09.
1
gave 0.090 g (58%) of the pure metal complex. H NMR (400 MHz,
∆-[Zn((-)-L2)](PF6)2 (9). The complex was prepared by dissolving
(-)-L2 (0.050 g, 0.058 mmol) in hot ethanol (100 mL) along with ZnCl2
(0.008 g, 0.058 mmol) and refluxing for 4 h. The solvent was removed
by rotary evaporation, and the resulting white solid was dissolved in
50 mL of water and then treated with 1.00 g of NH4PF6 (dissolved in
5 mL of water) to precipitate the complex. Subsequent filtering and
recrystallization via vapor diffusion of acetonitrile/ether gave 0.040 g
d6-acetone): δ 2.08 (s, 9H), 2.90 (m, 3H), 3.00 (m, 3H), 3.15 (m, 3H),
3.38 (m, 3H), 6.63 (s, 3H), 7.73 (t, J ) 1.10 Hz, 3H), 8.37 (t, J ) 7.87
Hz, 3H), 8.41 (d, J ) 4.76 Hz, 3H), 8.52 (d, 8.06 Hz, 3H), 8.76 (d,
J ) 8.05 Hz, 6H). 13C NMR (125 MHz, d6-acetone): δ 16.90, 29.50,
30.82, 123.26, 123.58, 127.59, 134.08, 134.89, 141.00, 142.06, 142.61,
147.06, 148.39, 148.64, 149.57. MS (m/z; ESI): 876 (17%, M2+
-
PF6-), 365 (100%, M2+ - 2PF6-). Anal. Calcd for C45H42N6Zn(PF6)2‚
1.5H2O: C, 51.51; H, 4.32; N, 8.01. Found: C, 51.42; H, 3.96;
N, 7.82.
1
(57%) of pure metal complex. H NMR (500 MHz, d6-acetone): δ
0.34 (s, 9H), 1.47 (s, 9H), 1.91 (d, J ) 10.38 Hz, 3H), 2.25 (s, 9H),
9
212 J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007

Controlling Helicity of Ru(II) and Zn(II) Complexes
A R T I C L E S
Photophysical Measurements. All photophysical measurements
were carried out in acetonitrile solution. UV-visible spectra were
recorded at room temperature in a 1.0-cm quartz cuvette using a
Hewlett-Packard 8453 spectrometer. CD spectra were collected by an
AVIV-62DS circular dichroism spectrometer and measured at room
temperature in a 1.0-cm quartz cuvette. Data points were collected at
1-nm intervals using averaging times of 1 s/nm with a spectral
bandwidth of 1.5 nm and sample concentrations of 25 µM. Optical
rotation data were obtained by a PE model 341 polarimeter with a tube
length of 1 dm.
Emission spectra were recorded using a Jobin-Yvon Fluorolog-3
spectrometer equipped with a double monochromator and a Hamamatsu-
928 photomultiplier tube (PMT) as the detector. Excitation occurred
at 450 nm for the Ru(II) compounds and at 315 nm for the Zn(II)
complexes.
Excited-state lifetimes were measured by exciting the samples at
337 nm with an N2 laser (Laser Science, Inc., VSL-337LRF, 10-ns
pulse) or by the fourth harmonic (266 nm) of a Nd:YAG (Continuum
Minilite) laser. The apparatus was connected to the Jobin-Yvon
Fluorolog-3 spectrometer, utilizing the emission monochromator and
PMT detector. The detector was in turn connected to a Tektronix TDS
3032B digital phosphor oscilloscope.
Circularly Polarized Luminescence Spectroscopy. The emission
of the samples was detected at a 90° angle to the excitation in a Jobin-
Yvon Fluorolog-3 and was passed through a photoelastic modulator
(Hinds International PEM-90, operating at a modulation frequency of
50 kHz), followed by a linear polarizer. The light was then incident
upon the emission monochromator. A Hamamatsu-928 PMT in
conjunction with a gated photon counter (Stanford Research SR 400)
was used to acquire the CPL spectrum.27
CPL measurements of the Ru(II) compounds were made over several
15-h periods. The photon counter gates were open for 30% (6 µs) of
the duty cycle of the PEM; two gates were used, each centered around
the extrema of the sinusoidal modulation cycle. The excitation light
for the Ru(II) complexes was 450 nm, with a slit width of 10 nm, and
the emission monochromator had a slit width of 6 nm.
Figure 1. (a) 4,5-Pinene bipyridyl ligand synthesized from myrtenal by
von Zelewsky and co-workers. (b) 5,6-Pinene bipyridyl ligand synthesized
from pinocarvone. (c) 4,5-Pinene bipyridyl ligand reported in this article
and synthesized from iso-myrtenal. The acidic protons in each molecule
are highlighted in red.
metals,2 so our synthetic strategy was focused on modifying
the 4,5-pineno-2,2′-bipyridine. Apart from the chiral nature of
the substituents, one of the key features of the pineno-2,2′-
bipyridines is the customizability provided by the acidic -CH2-
group found on the pinene ring (highlighted in red in Figure
1). This group can be easily deprotonated with a base such as
LDA and stereospecifically substituted with a variety of
electrophiles.
In previous work involving the 4,5-substituted complexes,
myrtenal was used as a precursor for the Kro¨hnke step, which
finalizes the bipyridine synthesis.14 The ligand can be depro-
tonated and substituted at the 7 position (Figure 1a), a location
that leads to two major drawbacks: (1) because the reactive
site faces the opposite direction to the chelating diimine moiety,
adding substituents to the ligand is unlikely to cause any
improvement to enantioselective catalytic processes, and (2)
geometric constraints make it extremely difficult to form a caged
metal complex using a simple linker between the three bipyridyl
subunits; to successfully form the complex, long chain spacer
groups containing ether linkages have been required.10
To address these issues, we endeavored to synthesize a
modified 4,5-pineno-2,2′-bipyridine ligand with the acidic
-CH2- protons located in a more advantageous location. Our
synthetic pathway resulted in the rotation of the pinene moiety,
thus moving the acidic methylene group to position 10 (Figure
1c, bottom). Following deprotonation and substitution with a
suitable cap, a hemicage ligand that was capable of coordinating
to a divalent transition metal ion was successfully prepared. This
approach necessitated the synthesis of a myrtenal isomer
precursor, which possesses a carboxaldehyde group at position
3 (Figure 1c, top) as compared to the myrtenal case where the
carboxaldehyde is attached at position 2 (Figure 1a, top).
Figure 2 outlines the synthesis of the chiral, pinenobipyridyl
hemicage ligands. The process consists of six synthetic steps
and is highlighted by a Kro¨hnke bipyridine synthesis.26 Ligands
(+)-L2 and (-)-L2 differ from the L1 ligands by the presence
The CPL measurements for the Zn(II) compounds were taken over
a 2.5 h period. The excitation was at 315 nm with a slit width of 5 nm,
and the emission monochromator had a slit width of 5 nm.
Owing to the difficulty in calculating absolute emission intensities,
the ratio of the differential emission intensity (∆I) to the total intensity
(I) is reported as the emission dissymmetry ratio (gem) in eq 1:
IL - IR
1/2I 1/2(IL + IR)
∆I
gem(λ) )
)
(1)
Here, IL and IR refer to the left and right circularly polarized
luminescence, respectively.
To ensure that there was no bias toward one type of circularly
polarized light, a solution containing racemic [Ru(L3)](PF6)2 was
measured. A slight bias was found, as the average value of gem was
-7.9 × 10-5. To adjust for this, ∆I for all CPL measurements was
corrected using eq 2.
7.9 × 10-5
∆Icorr ) ∆Iexp
+
Iexp
(2)
2
Results and Discussion
Synthesis. The chiral pinene ring can be fused to the 2,2′-
bipyridine ligand in either the 4,5 (Figure 1a,c) or the 5,6
position (Figure 1b). Because of strong steric crowding near
the nitrogen donor atom on the pyridine ring, 5,6-substituted
2,2′-bipyridines do not bond well to octahedral-coordinating
(27) Schippers, P. H.; van den Beukel, A.; Dekkers, H. P. J. M. J. Phys. E
1982, 15, 945-950.
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J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007 213

A R T I C L E S
Oyler et al.
Figure 2. Synthetic pathway for the chiral, pbpy caged ligands. The path shown is that of (-)-L1 and (-)-L2; ligands (+)-L1 and (+)-L2 used precursors
possessing the inverse stereochemistry. An achiral bpy caged ligand, L3, is also shown at right; upon complexation to a metal ion, the resulting compounds
from this ligand form in a racemic mixture of ∆ and Λ isomers.
Table 1. Photophysical Properties of Ru(II) Compounds
of a mesityl-linking group used in place of the simple phenyl
linker. It is important to note that the novel structure of the
absorbance
maximum
emission
maximum
(nm)
4,5-pineno-2,2′-bipyridine moiety achieved through this syn-
thetic pathway enables new strategies for tailoring these ligand
systems to a variety of applications similar to those of its two
cousins (Figure 1a,b). An achiral, hemicage bpy ligand (L3) was
also synthesized and is depicted in Figure 2 as well; upon
coordination to a metal ion, this ligand leads to the formation
of racemic complexes. The metal complexes of Ru(II) or Zn-
(II) were synthesized through complexation with Ru(DMSO)4Cl2
compound
MLCT (nm)
ΦPL
τ (µs)
[Ru(bpy)3](PF6)2
[Ru(L3)](PF6)2
∆-[Ru((-)-L1)](PF6)2
∆-[Ru((-)-L2)](PF6)2
451
454
468
468
605
595
603
603
0.062
0.237
0.010
0.084
1.14
3.28
0.12
0.87
to the racemic cage, [Ru(L3)](PF6)2. In the case of the mesityl-
capped diastereomers, ∆-[Ru((-)-L2)](PF6)2 and Λ-[Ru((+)-
L2)](PF6)2, the efficiency and lifetime are still comparable to
those of uncaged Ru(bpy)3](PF6)2 at 8.4% and 0.87 µs,
respectively. For the phenyl-capped diastereomers ∆-[Ru((-)-
L1)](PF6)2 and Λ-[Ru((+)-L1)](PF6)2, the efficiency and lifetime
are drastically reduced to 1.0% and 0.12 µs. The cause of these
reductions in relation to the racemic cage is not immediately
clear. The most likely explanation is the presence of the
additional hydrogen atoms associated with the pinene substit-
uents; these atoms increase the overall vibrational freedom of
the complex and can thus provide additional pathways for
nonradiative decay and inhibit luminescence. Additionally, it
is possible that the extra bulk of the pinene substituents can
slightly distort the complex and causes a subtle deviation from
the favored geometry, thus lowering the overall quantum
efficiency.
-
and ZnCl2, respectively, and isolated as the PF6 salt.
Ruthenium Caged Complexes. 1. Luminescence and
Absorption. Our initial studies of these caged bipyridyl
complexes were focused on the use of a simple mesityl linker
between three unmodified bipyridine units, similar to the
complexes studied by Beeston and co-workers.13,28 As in that
previous work, our racemic ruthenium complex, [Ru(L3)](PF6)2,
showed a large increase in excited-state lifetime and photolu-
minescent quantum yield over that of the uncaged [Ru(bpy)3]-
(PF6)2 species (Table 1). This increase was attributed to the
ligand sphere’s higher rigidity which reduces the rate constant
of vibrational, nonradiative decay (knr).
Along that line of reasoning, one would expect the hemicage
pineno-2,2′-bipyridine species to exhibit an even greater en-
hancement of the quantum yield and lifetime owing to the
presence of additional rigid substituents. However, what is
observed is a large decrease in both quantities in comparison
(28) Beeston, R. F.; Larson, S. L.; Fitzgerald, M. C. Inorg. Chem. 1989, 28,
4187-4189.
(29) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford,
CT, 2004.
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214 J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007

Controlling Helicity of Ru(II) and Zn(II) Complexes
A R T I C L E S
Figure 3. (Left) Isodesmic reaction of ∆-[Ru((-)-L1)]2+ with mesitylene to form ∆-[Ru((-)-L2)]2+ and benzene is shown. DFT calculations showed that
the mesitylene-capped complex is more highly strained by 14.7 kcal/mol. (Right) Structural isomer of ∆-[Ru((-)-L2)]2+ used as a stand-in for ∆-[Ru((-
)-L1)]2+ for purposes of ZPE comparison; red arrows indicate the location of the additional methyl groups. DFT calculations showed that the ZPE of the
mesityl-capped complex was higher by 1.7 kcal/mol.
To investigate the higher luminescent performance of the
mesityl-capped complexes over the phenyl-capped, density
functional theory (DFT) calculations at the B3LYP/LANL2DZ
level were carried out using Gaussian 03.29 An isodesmic
reaction was used to compare the amount of steric strain between
the phenyl-capped ∆-[Ru((-)-L1)]2+ and the mesityl-capped
∆-[Ru((-)-L2)]2+ (Figure 3, left). As expected, the more
crowded mesityl cap proved to be more strained by 14.7 kcal/
mol. Additionally, to understand better the enhanced perfor-
mance of the mesityl-capped complexes, the zero-point energies
(ZPE) of the compounds were also calculated. To compare the
energies for these two types of complexes by calculation, it is
necessary that the specific compounds to be compared are
isomeric. In this case, the phenyl-capped complex was converted
into a structural isomer of the mesityl-capped variety through
the addition of three methyl groups at the 5′ positions of the
bipyridine rings, a location that causes minimal steric hindrance
and thus should serve as an effective analogue (Figure 3, right).
These calculations demonstrate that the ZPE of the mesityl-
capped Ru complex is larger (1.7 kcal/mol) than that of the
phenyl-capped isomer. The higher ZPE corresponds to increased
rigidity of the compound, which should lower the rate of
nonradiative decay within the complex and can thus help to
explain the increase in quantum efficiency of the mesityl-capped
compounds.
2. Chiroptical Properties. CD spectra of the two mesityl-
capped Ru(II) cages are shown in Figure 5. The complexes each
show a Cotton effect at 282 and 299 nm, and the ∆ꢀ values are
of opposite sign for each enantiomer, clearly indicating that
predetermination of the helicity at the metal center was
successful.
Figure 5. CD spectra of ∆-[Ru((-)-L2)](PF6)2 (red) and Λ-[Ru((+)-L2)]-
(PF6)2 (blue). The samples were 25 µM in acetonitrile.
Figure 6 shows the CPL data for the phenyl-capped Ru(II)
cages, ∆-[Ru((-)-L1)](PF6)2 and Λ-[Ru((+)-L1)](PF6)2. At 600
nm, the values of gem are equal to -7.3 ((0.6) × 10-4 and 7.7
((0.6) × 10-4 for the ∆ and Λ, respectively. These complexes
show a slight decrease in gem as wavelength increases, consistent
with work by Gafni and Steinberg conducted on [Ru(bpy)3]2+
complexes.30
The absorption and emission spectra of the ruthenium
complexes are displayed in Figure 4. The emission spectra of
Ru(bpy)3(PF6)2 and the pinenobipyridyl complexes are very
similar with emission maxima at 605 and 603 nm, respectively.
The absorption spectra of the complexes are also reasonably
similar, with a notable exception occurring in the metal-to-ligand
charge transfer (MLCT) region between 350 and 500 nm. The
pinenobipyridyl complexes are all red-shifted to 468 nm from
the 451-nm absorption observed for Ru(bpy)32+, further sug-
gesting a slight distortion of the ligand sphere.
Figure 6. CPL spectra (solid lines) and emission dissymmetry values
(dashed lines) of ∆-[Ru((-)-L1)](PF6)2 (blue) and Λ-[Ru((+)-L1)](PF6)2
(pink). Samples were 25 µM in acetonitrile and degassed prior to collection
of their spectra (excited at λ ) 450 nm).
Figure 4. Combined absorption and uncorrected emission spectra of Ru-
(II) hemicage complexes.
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J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007 215

A R T I C L E S
Oyler et al.
The mesityl-capped Ru(II) cages, ∆-[Ru((-)-L2)](PF6)2 and
Λ-[Ru((+)-L2)](PF6)2, also show similar spectra, with values
of gem equaling -5.3 ((0.2) × 10-4 and 5.5 ((0.3) × 10-4 for
the ∆ and Λ isomers, respectively, as seen in Figure 7. Although
the signal varies, it is important to note that opposite CPL signals
were recorded for each set of diastereomers, indicating that the
CPL is caused by the arrangement of the ligands around the
metal center. In addition, the gem values for our Ru(II) cages
are consistent with those of previously reported complexes that
were separated through cocrystallization.27,31
form [Zn(L3)](PF6)2 drastically affected the emission spectrum
of the complex. The previously strong fluorescence in the UV
region becomes almost negligible, while a new, broad peak
centered around 485 nm is observed instead. This emission
proved to be phosphorescence, based on the relatively long-
lived excited-state lifetime (τ ) 0.089 µs) and the observance
of quenching by oxygen, as shown by the linear increase of
I0/I as the concentration of O2 was increased. The quenching
constant (kq) was calculated to be 1.5 × 109 M-1 s-1 through
the Stern-Volmer equation (eq 3), where [Q] is the concentra-
tion of the quencher, O2:
I0/I ) 1 + kqτo[Q]
(3)
Because Zn(II) complexes are d10, it can be assumed that this
transition does not involve the metal orbitals and is of a triplet
ligand-centered (3LC) nature. The shift in the energy maximum
from the fluorescent tris-bipyridine complex (λmax ) 326 nm)
to the phosphorescent cage complex (λmax ) 485 nm) amounts
to 1.24 eV, comparable to singlet-triplet energy gap values
given by Turro for aromatic molecules possessing π structure
similar to that of bipyridine.32 Photophysical measurements for
the Zn(II) complexes are summarized in Table 2.
Table 2. Photophysical Properties of Zn(II) Compounds
absorbance
maximum
(nm)
emission
maximum
(nm)
Figure 7. CPL spectra (solid lines) and emission dissymmetry values
(dashed lines) of ∆-[Ru((-)-L2)](PF6)2 (red) and Λ-[Ru((+)-L2)](PF6)2
(blue). Samples were 25 µM in acetonitrile and degassed prior to collection
of their spectra (excited at λ ) 450 nm).
compound
ΦPL
τ (µs)
[Zn(bpy)3](PF6)2
[Zn(L3)](PF6)2
∆-[Zn((-)-L2)](PF6)2
295
301
302
326
485
470
0.571
0.069
0.108
<0.010
0.089
0.074
Zinc Caged Complexes. 1. Luminescence and Absorption.
As was done for the Ru(II) case, a racemic caged bipyridyl
complex from Zn(II), [Zn(L3)](PF6)2, was synthesized and its
photophysical properties were compared to those of the uncaged
complex. For the parent [Zn(bpy)3](PF6)2 complex, an intense
emission was observed in the UV region at 326 nm (Figure 8).
A possible rationalization for the nearly complete switch from
singlet emission to triplet emission is the addition of the mesityl
cap itself. Hexa-alkyl substituted benzenes have previously been
shown to possess a triplet excited state similar in energy33 to
that of 2,2′-bipyridine.34,35 Therefore, the presence of this
additional triplet state provided to the complex by the mesityl
cap offers another possible radiative pathway through which
the compound can return to the ground state. Assuming good
vibrational overlap between the excited singlet state and the
various triplet states, intersystem crossing between these levels
could be strongly encouraged and thus cause the switch to
phosphorescent emission. Additionally, the greater rigidity of
the caged ligand sphere could prevent significant rearrangements
of the excited states and assist in providing good vibrational
overlap and thereby increase the rate constant for intersystem
crossing, kisc. This enhancement of phosphorescence could be
of use to the field of organic light emitting devices (OLEDs),
seeing as current emphasis in OLED design has been on the
use of molecules emitting from the triplet state. Very high
efficiency devices are thought to only be possible through the
use of phosphorescent emission, which should theoretically
Figure 8. Combined absorption (solid lines) and emission (dashed lines)
of [Zn(bpy)3](PF6)2 (blue) and racemic [Zn(L3)](PF6)2 (black). A Stern-
Volmer plot of the effect of diatomic oxygen quenching is depicted in the
inset. No quenching was observed for the fluorescent [Zn(bpy)3](PF6)2,
whereas the phosphorescence of [Zn(L3)](PF6)2 was strongly quenched.
(31) Blok, P. M. L.; Cartwright, P. S.; Dekkers, H. P. J. M.; Gillard, R. D.
J. Chem. Soc., Chem. Commun. 1987, 1232-1233.
(32) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Mill Valley, CA, 1991.
(33) Shizuka, H.; Ueki, Y.; Iizuka, T.; Kanamaru, N. J. Phys. Chem. 1982, 86,
3327-3333.
(34) Vinodgopal, K.; Leenstra, W. R. J. Phys. Chem. 1985, 89, 3824-3828.
(35) Ikeyama, T.; Okabe, N.; Azumi, T. J. Phys. Chem. A 2001, 105, 7144-
7150.
This emission appears to be fluorescent based on the short-
lived (<10 ns) excited-state lifetime and the resistance to oxygen
quenching (Figure 8, inset). The addition of the mesityl cap to
(30) Gafni, A.; Steinberg, I. Z. Isr. J. Chem. 1977, 15, 102-105.
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216 J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007

Controlling Helicity of Ru(II) and Zn(II) Complexes
A R T I C L E S
allow all injected electron-hole pairs to become triplet excitons
capable of light emission.36
transition in this region. This transition has a gem of -2.4 ((0.1)
× 10-3 and 2.4 ((0.1) × 10-3 at 460 nm for the ∆ and Λ
isomers, respectively; this corresponds to light that is 0.12%
circularly polarized. To our knowledge, these are the first
reported CD and CPL spectra for luminescent Zn(II) complexes
with predetermined configuration at the metal center.
The combined emission and absorption spectra of the mesityl-
capped Zn(II) pinenobipyridine cages are shown in Figure 9.
The emission is a single, broad peak of blue-white luminescence
centered around 470 nm. As with [Zn(L3)](PF6)2, lifetime
measurements (Table 2) and oxygen quenching studies of the
complexes indicate that the luminescence is phosphorescent in
nature. The quenching constant was 1.3 × 109 M-1 s-1, a value
smaller than that observed for the racemic compound; this result
can be rationalized by the greater steric bulk provided by the
pinene units which could partially shield the complex from the
oxygen quencher.
Figure 11. CPL spectra (solid lines) and emission dissymmetry values
(dashed lines) of ∆-[Zn((-)-L2)](PF6)2 (green) and Λ-[Zn((+)-L2)](PF6)2
(orange). Samples were 25 µM in acetonitrile and degassed prior to
collection of their spectra (excited at λ ) 315 nm).
Conclusions
Figure 9. Combined absorption and emission spectra of ∆-[Zn((-)-L2)]-
(PF6)2.
We have synthesized both enantiomers of a new, pinene-
substituted 2,2′-bipyridine ligand with broad potential applica-
tions in the fields of asymmetric catalysis and the stereoselective
synthesis of metal complexes. The ligand can readily be
substituted with a linking spacer to form a hemicaged species
capable of forming chiral metal complexes of Ru(II) and Zn-
(II) with complete selectivity, as was shown through both CD
and CPL measurements. Additionally, the Zn(II) complexes
were shown to display a heretofore unprecedented switching
from fluorescent to phosphorescent emission between the
2. Chiroptical Properties. The CD spectra for the Zn(II)
mesityl-capped chiral cages are shown in Figure 10. A Cotton
effect of equal and opposite relationship was observed at 316
nm for the diastereomers. It is noteworthy that the complexes
remained stable over the course of months with no observable
racemization.
2+
uncaged Zn(bpy)3 and the hemicaged species. Such an
enhancement of phosphorescent emission could lead to future
discoveries of efficient triplet emitters for use in ionic OLED
applications.
Acknowledgment. This work was supported by the National
Science Foundation (Career Award No. CHE-0449755) and a
Camille and Henry Dreyfus Foundation New Faculty Award.
F.J.C. acknowledges support by a National Science Foundation
Graduate Research Fellowship.
Supporting Information Available: Complete citation for ref
29 as well as NMR and mass spectrometry data for key metal
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 10. CD spectra of ∆-[Zn((-)-L2)](PF6)2 (green) and Λ-[Zn((+)-
L2)](PF6)2 (orange). The samples were 25 µM in acetonitrile.
Figure 11 displays the CPL data for the Zn(II) mesityl-capped
cage complexes. For each complex, the CPL is essentially
constant from 430 to 560 nm, indicating that there is only one
JA067016V
(36) Forrest, S. R. Org. Electron. 2003, 4, 45-48.
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J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007 217